151 
the distribution of the pulses with reference to the floods and the 
appearance of pulses during rising water suggest the operation of 
other factors than the one arising from contribution from back- 
waters. 
The pulse must be dependent to a large extent upon food supply 
of the organism, and a correlation between its periods of multiplica- 
tion and the pulses of its food, the chlorophyll-bearing organisms, is 
to be expected. A comparison of the seasonal distribution in 1898 
(Table I.) and the pulses of chlorophyll-bearing organisms (PI. IT.) 
reveals the fact that three of the A. cochlearis pulses coincide with 
those of the plants constituting their food, and the other three 
coincide in part only, the remainder of the chlorophyll-bearing 
groups reaching their culmination a week prior to that of the rotifer. 
In 1897 the three pulses of A. cochlearts which lie in the common 
period (Pl. II.) all culminate a week (in one case in part in fourteen 
days) after the maximum of the plants in question. In 1896, three 
pulses coincide and three follow in the subsequent collection; and 
in 1895, two coincide and two follow. Collections at daily intervals 
would be necessary to follow the correlation more accurately. It is 
probable from these juxtapositions and sequences in the A. 
cochlearis-algzee pulses that we are dealing with a food relation. 
Multiplication of algze leads to increase of Anurea, which, in turn, 
reduces the algze,and then itself declines until the food planktonts 
again increase. 
Anurea cochlearts is exceedingly variable in the length of the 
posterior spine, in the development and degree of curvature of the 
anterior spines, in the arrangement of the areas of the lorica, and in 
the degree of its ornamentation by small spinules. The separation 
of these varieties where every individual must be assigned to some 
one of them,is a matter of some difficulty owing to the presence of 
intergrading individuals. The characters which signalize var. 
hispida Lauterborn and var. irregularis Lauterborn are not quickly 
recognized under the conditions of rapid plankton enumeration, and 
no effort was made to trace their seasonal distribution in our plank- 
ton. Lauterborn’s var. macracantha was included with the type 
form—his var. typica—in our records. These two include those 
individuals with medium-sized and longer posterior spines. In our 
waters the variety macracantha is relatively rare, at least as figured 
by Lauterborn (98). Indeed, both the type and this variety consti- 
